EXPRESSION AND DEVELOPMENT OF A FUNCTIONAL PLASMALEMMAL 5-HYDROXYTRYPTAMINE TRANSPORTER BY THYROID FOLLICULAR CELLS

5-hydroxytryptamine (5-HT) is synthesized and secreted by thyroid para follicular (PF) cells. As all PF granules contain 5-HT, it is released whenever PF cells secrete. Because 5-HT stimulates follicular (F) cel ls and can modulate their response to TSH, 5-HT has been proposed to b e a paracrine PF to F cell transmitter. This role would require a thyr oid mechanism to rapidly inactivate 5-HT. A 5-HT transporter (SERT) in the plasma membrane of serotonergic neurons inactivates neuronal 5-HT . We thus tested the hypothesis that this molecule is expressed in the thyroid. Messenger RNA encoding SERT was demonstrated in both the hum an thyroid and a rat F cell line (FRTL-5). SERT immunoreactivity was d etected in rat F, but not PF, cells. Transporter-mediated uptake of[H- 3]5-HT by F cells arose early in development (E13 in mice) and was mai ntained in adult life in mice, guinea pigs, bats, and rats (FRTL-5 cel ls). These observations indicate that a functional SERT is expressed i n the thyroid, not by the 5-HT-secreting PF cells, but by their putati ve F cell targets.